The present invention relates to a reflective liquid crystal display device comprising first and second substrates between which liquid crystal material is disposed and electrodes provided on the substrates defining an array of display pixels, the first substrate having an array of optically reflective pixel electrodes each of which is connected to the output of a respective switching device carried on the first substrate and is provided on the surface of an insulating layer extending over the first substrate and covering the switching devices.
An example of such a display device is described in EP-A-0617310. In this device, a row and column matrix array of display pixels is provided, each of which is driven via an associated switch device in the form of a TFT (thin film transistor). The TFTs are carried on the surface of a first substrate together with sets of row, selection, conductors and column, data, conductors through which the TFTs are addressed for driving the display pixels. As in conventional active matrix LCDs using TFTs, each TFT is disposed adjacent the intersection between respective ones of the row and column conductors. The gates of all the TFTs associated with a row of display pixels are connected to a respective row conductor and the sources of all the TFTs associated with a column of pixels are connected to a respective column conductor. Unlike conventional active matrix LCDs, however, in which the individual pixel electrodes are arranged substantially co-planar with, and laterally of, the TFTs, the reflective pixel electrodes in this device are carried on an insulating film which extends over the first substrate and covers the TFTs and the sets of address conductors so that the pixel electrodes are positioned generally above the level of the TFTs and the address conductors. Each individual pixel electrode is connected to the drain electrode of its associated TFT through a respective opening formed in the insulating film directly over the drain-electrode. An advantage of this type of construction, in which the array of pixel electrodes and the array of TFTs are provided at different levels above the substrate surface, is that the pixel electrodes can be enlarged such that at two opposing sides they extend slightly over adjacent row conductors and at their two other opposing sides they extend slightly over adjacent column conductors rather than being sized smaller than the spacing between adjacent row conductors and adjacent column conductors with small gaps provided between each edge of the pixel electrode and the adjacent conductor, as in conventional display device arrangements. In this way, therefore, the pixel aperture is increased and in operation more light which passes through the liquid crystal layer and reaches the pixel electrode is reflected back to produce a brighter display output. Moreover, parts of a deposited metal layer which is patterned to form the reflective pixel electrodes can be left immediately overlying the TFTs during the patterning process so as to act as light shields for the TFTs to reduce photoelectric effects in the TFTs due to light incident thereon, thereby avoiding the need to provide black matrix material on the other substrate for this purpose as is usual. This other, transparent, substrate carries a continuous transparent electrode common to all pixels in the array and, in the case of a colour display, an array of colour filter elements corresponding to the array of pixels with each filter element overlying a respective pixel electrode.
In order to improve the reflection characteristics of the pixels, particularly the resulting intensity of light scattering in the direction perpendicular to the display panel with respect to light incident on the pixel electrode at any angle, the pixel electrodes in the display device of EP-A-0617310 are made undulating by forming the region of the insulating film underlying the reflective pixel electrode with a plurality of randomly arranged bumps so that the pixel electrode deposited thereon, and comprising a metal layer of substantially constant thickness, similarly has surface bumps. These bumps on the pixel electrode serve to scatter light so that a greater proportion of light incident on the electrode from any angle is reflected in a direction normal to the panel to increase pixel luminance. The bumps in the insulating film are themselves formed by patterning a deposited photoresist layer with the aid of a mask, light exposure and development to leave discrete dots of photoresist whose area and position are determined by the mask, and then depositing a further organic insulating layer over these dots. Thereafter a contact opening is formed at each pixel in the insulating film overlying the drain electrode of the TFT and a reflective metal layer is deposited which extends through these openings to contact the underlying drain electrodes and which is patterned to define the individual pixel electrodes.
The formation of the undulating pixel electrodes is thus complicated, involving the deposition and processing of a number of separate layers including photoresist and organic insulating films which add significantly to the complexity of manufacture. Importantly, it is necessary for the dots of photoresist material to be shaped appropriately to avoid sharp edges and the like so that suitably shaped bumps result at the surface of the pixel electrodes and also for the region overlying the drain electrodes to be kept free of bumps.
It is an object of the present invention to provide an improved reflective LCD of the kind described in the opening paragraph.
It is another object of the present invention to provide a reflective LCD of the aforementioned kind which is relatively simple to manufacture.
According to the present invention, there is provided a reflective LCD of the kind described in the opening paragraph which is characterised in that the pixel electrode is connected to the output of the switching device via a plurality of contact openings in the insulating layer at spaced locations over the area of the pixel electrode and having sloping sidewalls over which the pixel electrode extends. The plurality of contact openings serve to enhance the reflection characteristics of the pixel electrode. These contact openings result in depressions in the pixel electrode surface which, in comparison with the structure of EP-A-0617310, are effectively negative, or inverse, bumps but which behave in similar, light scattering, fashion for reflecting incident light. Moreover, the plurality of contact openings serves also to provide a degree of redundancy in the electrical connection between the pixel electrode and the switching device output.
The plurality of contact openings at each pixel location can be provided in simple manner, for example by patterning the insulating layer photolithographically using a mask to define the contact openings and their relative disposition. The layer may be etched or, in the case of the insulating layer comprising a photo-resist material, photodeveloped. It is to be noted that in the device of EP-A-617310, a single contact opening is provided in the insulating layer by a photolithographic process before the pixel electrode is deposited so as to enable electrical connection between the pixel electrode and the underlying drain electrode of the TFT to be established. The provision of a plurality of contact openings in the device of the present invention does not add significantly to the complexity of the processing in this manufacturing stage and can be accomplished without any additional processing operations being required.
Preferably, the plurality of contact openings are arranged regularly over a substantial part of the pixel electrode area, for example in a generally uniform row and column array occupying 50% or more of the overall area of the pixel electrode. The number, size, shape and relative disposition of the openings can be varied. For example, the openings may be generally circular or square. As a consequence of the openings being formed by a photolithographic method such as etching the sidewalls of the openings in the insulating layer will be sloping to some extent so that the shape of the resulting depressions in the subsequently deposited pixel electrodes will be tapering, e.g. generally conical in the case of circular openings. The exact shape of the depressions will depend though on the relative thicknesses of the insulating layer, and thus the depth of the openings, and the material of the pixel electrode layer as well as the width of the openings. If the pixel electrode layer is relatively thick then the resulting depressions formed therein will tend to smoothed out to some extent, for example in the nature of inverted domes. The openings may be spaced apart from one another so that substantially flat areas of the pixel electrode layer exist between adjacent openings or arranged close together so that the extent of the pixel electrode material between the depressions is reduced or minimised.
Depending on the number and position of the contact openings with respect to the switching device, connection between the pixel electrode and the output of the switching device may be achieved via respective, individual, electrically conductive tracks underlying the insulating layer and extending from the switching device. Preferably, however, an electrically conductive layer connected to the output of the switching device is provided beneath the insulating layer extending over a substantial part of the area of the pixel electrode, corresponding to the region in which the contact openings are formed. In the case of the switching device comprising a TFT, this conductive layer may be formed integrally with the drain electrode of the TFT, as an extension, from a single deposited layer. Again, comparing this with the display device of EP-A-0617310, it will be appreciated that the provision of this underlying electrically conducting layer requires no significant additional processing operations. Thus, the reflection property enhancing depressions in the pixel electrode of the device of the present invention can be provided in simple manner merely by modifying certain existing fabrication operations.
Preferably, the surface of this electrically conductive layer is rough so that after depositing the insulating layer and pixel electrode thereon the surface of the pixel electrode possesses a degree of roughness, providing surface asperities. Such roughness in the surface of the pixel electrodes in the regions around the contact openings assists in achieving desirable scattering reflection characteristics. The roughness of the conductive layer may be introduced deliberately or achieved as a natural consequence of fabrication processing by appropriately selecting the deposition conditions. In the case, for example, of the switching device comprises a polysilicon TFT having source and drain contacts of laser crystallised, n type, polysilicon, the n type polysilicon material inherently has a degree of surface roughness which may be adequate for this purpose. If a metal is used for this layer then depositing the metal in a substantially pure form will tend to create bigger grains, and hence roughness. Also, the deposited material of this underlying electrically conducting layer may be deliberately roughened by further processing in a known manner to provide this effect.
It will be appreciated that switching devices other than TFTs may be employed, for example two-terminal non linear switching devices such as MIMs or TFDs (thin film diodes). When using such devices it is necessary to provide only one set of address conductors, e.g. the row, selection, address conductors on the same substrate as the switching devices and reflective pixel electrodes, the other set of address conductor, e.g. the column, data, address conductors, being provided on the other substrate.
The display device may be a monochrome display device or a colour display device in which colour filter elements are provided on the other substrate, for example as described in EP-A-0617310.